Vision impairments such as myopia (near-sightedness), hyperopia (far-sightedness) and astigmatism can be corrected using eyeglasses or contact lenses. Alternatively, the cornea of the eye can be reshaped surgically to provide the needed optical correction. Eye surgery has become commonplace with some patients pursuing it as an elective procedure to avoid using contact lenses or glasses to correct refractive problems, and others pursuing it to correct adverse conditions such as cataracts.
With recent developments in laser technology, laser surgery is becoming the technique of choice for ophthalmic procedures. The reason eye surgeons prefer a surgical laser beam over manual tools like microkeratomes and forceps is that the laser beam can be focused precisely on extremely small amounts of ocular tissue, thereby enhancing accuracy and reliability of the procedure. These in turn enable better wound healing and recovery following surgery.
Different laser eye surgical systems use different types of laser beams for the various procedures and indications. These include, for instance, ultraviolet lasers, infrared lasers, and near-infrared, ultra-short pulsed lasers. Ultra-short pulsed lasers emit radiation with pulse durations as short as 10 femtoseconds and as long as 3 nanoseconds, and a wavelength between 300 nm and 3000 nm. Examples of laser systems that provide ultra-short pulsed laser beams include the Abbott Medical Optics iFS Advanced Femtosecond Laser, the IntraLase FS Laser, and the CATALYS Precision Laser System.
In laser systems for eye surgery, the quality of the laser beam is determined by how well the beam can be focused to a specific point, i.e., a circular area having a diameter typically of 1-2 microns, which is directly related to how well the beam can incise or ablate tissue. But, over time, the laser beam may fail to meet quality specifications due to optics misalignment, obscuration, or other failure modes. In this case, continued use of the laser system may result in cutting and ablation which is incomplete or degraded. Laser systems are therefore frequently tested to verify the beam quality. With many laser surgery systems, a beam quality test is performed every single day, well before the first patient is treated. A well-known beam quality test is performed by using the focused laser beam to make cuts in a test sample, such as a plastic sphere. The cuts are then inspected under magnification, and the beam quality is inferred from the characteristics of the cuts, such as the positions and the completeness of the cuts in the plastic sphere.
While this sample cutting technique may be relatively easily performed, it has several disadvantages. Initially, determining beam quality by inspecting cuts in a test sample is subjective and depends largely on the judgment of the inspector, typically an eye surgeon in a surgical facility, and not a laser system technician who may have better knowledge of the laser system. The sample cutting test is also an indirect qualitative measurement, rather than a direct quantitative measurement. In addition, no diagnostic information is provided when the inspector determines the system has failed the test. The sample cutting test also provides little or no information on changes in beam quality over time, which information may be useful in predicting an impending failure, or evaluating the cause of a failure.
Other techniques for measuring laser beam quality, such as the so-called bubble threshold test, have also been used, with varying degrees of success. But, these types of tests require more extensive equipment, time, and expertise. Thus, although these tests are useful in laboratory or factory settings, they are not well suited for daily use by an eye surgeon in a surgical facility. Consequently, engineering challenges remain in measuring laser beam quality in laser eye surgery systems.
In many laser eye surgery systems, the laser beam is directed via a scanning mirror. Position sensors associated with the scanning mirror can sense the position of the scanning mirror. If the laser eye surgery system is not properly calibrated, however, the actual position of the laser beam on or in the treatment volume of the eye may not correspond sufficiently precisely with the position information from the position sensors. As a result, engineering challenges also remain in designing improved techniques for calibrating laser eye surgery systems.
Therefore, there is a need for new and improved methods for measuring beam quality in laser eye surgery systems.